I’ve created two graphs which run over a 28-day period, and I’ve set “Display - Time Regions” to do a Fill on “Mon to Fri”, so that I see the distinction between weekdays and weekends.
If the start (left hand end of the X-Axis) of the graph is during a weekend, I get a fill pattern starting on the first Monday extending to the first Friday, then a second one from the second Monday to the second Friday, and so on. Since the graph starts in a weekend and extends for 28 days, it ends in a weekend too.
However, if the graph starts during a week, there is no fill up to the end of the first Friday; the first filled section starts on the first displayed Monday and extends to the end of that week, which is the second displayed Friday.
The right hand end of the graph does have a filled section from the final Monday to the end of the graph, though (as I would expect).
So, I believe this to be a bug in the fill algorithm, in that it correctly starts a partial week’s fill at the right hand end of the graph, but does not display a partial week’s fill at the left hand end of the graph.
Grafana 5.4.4 and 6.2.2